ABSTRACT
The completion of the sequencing of human and other genomes has provided a vast amount of material for gene expression and gene function studies. Recombinant protein expression plays an important role in these activities. During the past few years, alternative expression systems based on both prokaryotic and eukaryotic vectors have been engineered. Two excellent, albeit now to some extent outdated, reviews have previously been published on heterologous expression of integral transmembrane proteins1 and more specifically G protein-coupled receptors (GPCRs).2 In a more recent review, the focus is on large-scale production of GPCRs applying various expression systems.3 Bacterial vectors have often been favored due to their simplicity, low cost, and scalability. In this context, many soluble proteins have been efficiently expressed in bacteria, especially in Escherichia coli, independent of whether their origin was prokaryotic or eukaryotic. Integral membrane proteins have, however, presented a different challenge. The higher complexity of membrane proteins, as compared with soluble proteins, has made their structural characterization significantly more difficult. Despite that, relatively good success has been obtained for the expression of various bacterial membrane proteins,4 which has formed the basis for the generation of several high-resolution structures for
prokaryotic targets.5 In contrast, expression of eukaryotic and especially mammalian membrane proteins in bacteria has been less successful, although recently several GPCRs have given reasonable yields when expressed in either E. coli inclusion bodies6 or membranes.7 A more detailed description of bacterial expression can be found in Chapter 3 through Chapter 5.
